The Evolution of the Horse. 
Ill 
extreme. In many organs, but especially in the limbs and 
teeth, we find the strongest evidence of two opposing principles 
striving against each other for the mastery in fashioning their 
form and structure. "We find heredity, or adherence to a 
general type derived from ancestors, opposed by special modifi- 
cations of, or deviations from, that type, and the latter generally 
getting the victory. The various specialisations, evidently in 
adaptations to purpose, will be thought by many to be the result 
of the survival, in the severe struggle for existence, of what is 
fittest for the purpose to which it is to be applied. This may or 
mav not be the explanation, but the interest of the study of such 
an animal as the horse will be increased tenfold by the conviction 
that there is some true, and probably discoverable, causation for 
all its modifications of structure, however far we may yet be from 
the true explanation of the methods by which they have been 
brought about. 
The anatomy and history of the horse are, moreover, often 
taken as a test case of the value of the theory of evolution, or 
the transmutation of animal forms one from another with the 
advance of time. The evidence in favour of or against the 
theory may in this case be derived from two distinct sources : 
I. The structure of existing horses ; II. The past history of 
the race as revealed by fossil remains. 
L By far the most interesting portions of the organisation 
of existing horses from this point of view are the various 
rudimentary, and apparently useless, structures which occur in 
several parts of its body — structures which correspond with some 
which are fully developed in other animals, but which in the 
horse are so reduced in size or altered in character as to be of 
little, if any, use in its economy. In tracing the history and 
affinity of animals, rudimentary organs are looked upon by 
naturalists as far more important and interesting than highly- 
developed or functional parts. As Darwin says, they " may be 
compared with the letters of a word, still retained in the spelling, 
but become useless in the pronunciation, but which serve as a 
clue to its derivation. 1 On the view of descent with modifica- 
tion, we may conclude that the existence of organs in a rudi- 
mentary, imperfect, or useless condition, or quite aborted, far 
from presenting a strange difficulty, as they assuredly do in the 
old doctrine of creation, might even have been anticipated in 
accordance with the views here explained."' 
II. It is, however, to the ancestral history, as disclosed by 
palaeontology, or the study of fossil remains, that we must look 
1 As, for example, the b in 1 debt ' and 1 doubt." 
